1. Field of the Invention
The general field of the invention is that of the synthesis of silicone resins, in particular that of the synthesis of MQ type silicone resins.
2. Description of Related Art
Resins are polymers of relatively low molecular weight having a three-dimensional structure. They have many uses and they find applications in the fields of self-adhesives (for modifying the peel strength), as anti-foaming agents, mold-release additives, paint additives and in many other fields, affecting a large number of industries. Siloxane resins are known which are products that are commercially available and that are used in the preparation of silicone-based products such as adhesives and anti-foaming products. Such resins are sometimes denoted by the term “MQ resins” due to the presence of monovalent siloxyl units (M) and tetravalent units (Q).
They have very varied structures which depend on the amount of trifunctional T or tetrafunctional Q units introduced into the polymer during the manufacture. It is recalled that in silicone chemistry terminology, the siloxane units M, D, T and Q are defined as follows:

The backbone of MQ resins results from the polycondensation of a liquid silicate, the source of Q units, onto which monofunctional chlorosilanes, the source of M units, are subsequently grafted by hydrolysis of the silanol functional groups that have not polycondensed.
The synthesis of MQ silicone resins occurs, in a first step, by the formation of polysilicic acid. The hydrolysis reaction between sodium silicate and hydrochloric acid is the first step of the process for synthesis of silicone resins. This step consists of building the backbone of the resin by formation of polysilicic acid macromolecules onto which reactive or non-reactive units are subsequently grafted, giving it certain properties. The groups which may be grafted onto the macromolecule are varied and are involved in the usage properties of the resin.
Among the types of non-reactive groups, mention may be made of:                methyl groups which are the source of the non-stick properties, the hydrophobicity and the surface harness; and        phenyl groups which provide temperature resistance, high-temperature flexibility and compatibility with organic products.        
Among the types of reactive groups (which enable crosslinking on application) mention may be made of:                hydroxyl groups, which enable crosslinking by condensation at ambient temperature in the presence of a catalyst or in an alkali medium;        alkoxy groups which hydrolyze to hydroxyl groups at ambient temperature and in the presence of moisture; and        vinyl groups which react at moderate temperature by addition reaction in the presence of platinum.        
These resins, in the absence of solvent, are solid and of white appearance and are handled in a solvent phase either in xylene, toluene or white spirit or in a silicone oil. In solution or in powder form, these compounds are tacky unlike silicone oils which are lubricating instead.
The known processes for synthesizing silicone resins are batch processes, that is to say that at first sodium silicate is run into a suitable reactor over a “bottoms” of dilute hydrochloric acid in order to form the polysilicic acid (PSA). This step is generally carried out in a stirred reactor operating in semi-sealed mode.
Next, a step is carried out in order to slow down the polymerization reaction by addition of an alcohol (quenching step). Next, an organic solvent is then run in which makes it possible to extract the functionalized resin. The functionalization is carried out by running in chlorosilanes, comprising a variable number of chlorines and alkyl groups, which react with the residual silanol functional groups of the polysilicic acid. The functionalization, via the choice of chlorosilanes used, makes it possible to target the application properties of the resin. Again, a new solvent is run in in order to terminate the extraction of the resin in the organic phase. This complementary addition of solvent has the objective of favoring the decantation of the organic phase (containing the resin) from the aqueous phase. Finally, an advancement step may optionally be carried out that consists in reducing the number of residual silanol functional groups after the hydrolysis step is carried out. A potassium hydroxide solution is run in, leading to a consumption of the residual silanol functional groups. The excess of potassium hydroxide is generally neutralized by addition of an acid into the medium, for example phosphoric acid, and the whole mixture is filtered.
Examples of such processes are described, for example, in patents U.S. Pat. No. 2,676,182 (Daudt et al.) and U.S. Pat. No. 2,814,601 (Currie et al.).
However, during the polycondensation step, running in the sodium silicate over the acid bottoms is a step that is tricky to carry out on an industrial scale. A slight variation in the run in time, the temperature, and the concentrations of reactants have serious consequences for the quality of the polysilicic acid formed, backbone of the final resin. This is because, according to the conditions for carrying out the synthesis, localized gels, or even setting of the medium, are observed in the reactor. This step is a determining factor in the quality of the final resin obtained.
Such processes therefore have a not insignificant variability per batch of MQ resins manufactured and only offer a little flexibility in the operating parameters such as the flow rates for running in the reactants and the amount of reactants. Furthermore, they do not impart flexibility as regards the possibility of preparing MQ resins of low molecular weight.
More recently, Patent Application EP 1 113 036 describes a process for the continuous preparation of silicone resins comprising, in addition, the following steps:    a) a continuous polymerization of sodium silicate, in an aqueous medium, in the presence of an acid to form a silica sol rapidly followed by a continuous step of quenching by addition of an alcohol; and    b) a continuous capping by addition of an organosilane in a water/organic solvent reaction mixture.
This reference also describes a “semi-continuous” configuration where the polycondensation reaction is carried out continuously in a tubular reactor with in-line injection of sodium silicate or in a loop reactor, then the downstream steps are carried out in a sealed reactor. It is indicated that the molecular weight of the silica sol depends on the reaction temperature and also on the residence time of the reactants: sodium silicate/acid. Thus, the synthesis of low-viscosity resin is achieved by ensuring a short residence time (less than one minute) of the sodium silicate/acid reactants. If not, for the synthesis of higher molecular weight resins, the recommended residence time for the preparation of polysilicic acid is between 1 and 5 minutes. The examples of reactors described that make it possible to achieve this constraint linked to the residence time of the reactants necessary for the synthesis of the polysilicic acid are:                a stirred column having several stages (a Scheibel type column, preferred configuration, each stage corresponding to one step of the process);        a tubular reactor comprising static mixers (plug-flow static mixer reactor), and        a loop where the hydrochloric acid circulates with injection of sodium silicate (packed plug-flow reactor).        
It should be noted that when low-viscosity resins are desired, the process described requires a rapid addition of the quenching agent (alcohol) for the polymerization reaction, this being in order to prevent too high a polycondensation of the polysilicic acid and therefore a rapid increase in the viscosity. Another lever for controlling the viscosity of the MQ resin obtained is to act on the duration of the functionalization step (see, for example, examples 2 and 4).
Although the process briefly described above provides a satisfactory technical solution to the difficulties explained above, it is however, desirable to provide improvements, especially as regards the control of the operating conditions of the polycondensation step leading to the formation of polysilicic acid. This is because, the process described above and those of the prior art do not allow great flexibility on an industrial level since during the polycondensation step leading to the formation of polysilicic acid, the flow rate and the concentration must be carefully controlled so as to keep the molar ratio [SiO2 of the sodium silicate/acid] in a restricted value range in order to prevent the reaction medium from setting as a gel.
It is also known that any variation, even a minor variation, in the pH can modify the equilibrium conditions of the polycondensation reaction. Furthermore, sodium silicate is extremely sensitive to concentration phenomena and, consequently, any variation, even a minor variation, of the flow rates of the reactants can modify the dilution of the silicate in the medium.